Human immunodeficiency virus (HIV) has been identified as the etiological agent of human immunodeficiency syndrome (AIDS). At present, there is no effective vaccine against this disease. The object of this investigation is to characterize HIV antigens, determine the targets of humoral and cell mediated immunity and to use this information to develop candidate vaccines. These viruses have been used as live vaccines to immunize animals, to synthesize subunit proteins, and to make targets for cytotoxic T cells. Our finding that HIV infected individuals have circulating cytotoxic T cells directed to conserved epitopes within reverse transcriptase, as well as gag and env, may be significant for vaccine development.